The invention relates generally to optical scanners and, more particularly, to methods and apparatus for controlling the airflow produced by a rotating optical component.
Some printers and scanners employ optical sources such as lasers to repeatedly scan an image field. For example, the light beam may sweep out a strip on an object being scanned while the surface of the object moves past the scanning beam. The object may be a document or a surface covered with a photo sensitive medium such as unexposed film or a laser printing drum. To variably expose the photo sensitive medium, the scanning beam may be modulated. To produce the repeated sweep scans, the scanner may employ a rotating mechanical device which sweeps an incoming optical beam along the scan strip at least once during each rotation period.
FIG. 1 schematically illustrates a portion of a light scanner 5. A light source 10 produces an incoming visible or infrared light beam 15. A mirror 20 has one or more reflecting facets 50, 55 that deflect the incoming light beam 15, towards a focusing lens system 25, which produces a scan beam 30. The scan beam 30 sweeps out a scan strip 35 in the image field. To produce the sweeping action of the scan beam 30, a motor 40 rotates the mirror 20 at a pre-determined angular velocity. During a portion of the rotation period of the mirror 20, the scan beam 30 sweeps out the strip 35.
For a high resolution scan, the strip 35 is a very fine, for example, less than about 1/1000 of an inch wide. To scan an image field rapidly with such fine scan beams, the motor 40 typically turns the mirror 20 at a high frequency, for example, 20,000 revolutions per minute (RPM).
In high-resolution printing, very small variations in the scan beam 30 can produce image defects detectable by the human eye. Time-varying refractive indices along the incoming and/or deflected light beams 15, 45 can displace the scan beam 30 by as much as the distance between successive scan strips 35 on the object being scanned (not shown) thereby causing the image defects. Small variations in the local air density can produce a changing refractive index sufficient to cause such image defects in highresolution devices.
The rotating mirror 20 generates pressure waves in the adjacent air. A first source of such waves is a time-changing form, which an axially non-symmetric and rotating mirror presents to adjacent air, in each direction about the rotation axis. For example, the facets 50, 55 destroy full axial symmetry of the mirror 20. A second source of pressure waves is viscous drag caused by the rotational velocity of the mirror 20. Viscous drag can produce pressure waves even if the mirror 20 is replaced by a very symmetric beam deflector (not shown) such as a hologon or holographic deflector. For high-resolution scanners, the high rotational speed of the mirror 20 can produce turbulent and/or vortex airflow in the work space surrounding the mirror 20. The vortex or turbulent air flow presents a time-dependent air density and refractive index for the incoming and reflected beams 15, 45. The turbulent airflow can noticeably degrade the quality of an image scan by introducing image defects known as turbulence banding.
One method of eliminating image defects such as turbulence banding entails evacuating the work space around rotating optical parts such as the mirror 20 of FIG. 1. Evacuating the work space generally requires a complex and costly vacuum source. The vacuum source may also add significant bulk to the optical scanner. Thus, operating the rotating mirror in a vacuum may not be a convenient or cost-efficient method for eliminating defects such as turbulence banding.
The present invention is directed to overcoming, or at least reducing, one or more of the problems set forth above.